Make slop optional when parsing `span_near` query (#25677)

The slop parameter defaults to 0 in the Lucene SpanNearQuery, so we can set it
to this default value also and don't have to require it being specified in the
query when using the Rest API. Leaving `slop` a ctro arg in the Java API as it
should normally be specified and we can keep it `final` that way.

Closes #25642

core/src/main/java/org/elasticsearch/index/query/SpanNearQueryBuilder.java

@@ -45,6 +45,8 @@ public class SpanNearQueryBuilder extends AbstractQueryBuilder<SpanNearQueryBuil
 
     /** Default for flag controlling whether matches are required to be in-order */
     public static boolean DEFAULT_IN_ORDER = true;
+    /** Default slop value, this is the same that lucene {@link SpanNearQuery} uses if no slop is provided */
+    public static int DEFAULT_SLOP = 0;
 
     private static final ParseField SLOP_FIELD = new ParseField("slop");
     private static final ParseField CLAUSES_FIELD = new ParseField("clauses");
@@ -145,8 +147,8 @@ public class SpanNearQueryBuilder extends AbstractQueryBuilder<SpanNearQueryBuil
 
     public static SpanNearQueryBuilder fromXContent(XContentParser parser) throws IOException {
         float boost = AbstractQueryBuilder.DEFAULT_BOOST;
-        Integer slop = null;
-        boolean inOrder = SpanNearQueryBuilder.DEFAULT_IN_ORDER;
+        int slop = DEFAULT_SLOP;
+        boolean inOrder = DEFAULT_IN_ORDER;
         String queryName = null;
 
         List<SpanQueryBuilder> clauses = new ArrayList<>();
@@ -189,10 +191,6 @@ public class SpanNearQueryBuilder extends AbstractQueryBuilder<SpanNearQueryBuil
             throw new ParsingException(parser.getTokenLocation(), "span_near must include [clauses]");
         }
 
-        if (slop == null) {
-            throw new ParsingException(parser.getTokenLocation(), "span_near must include [slop]");
-        }
-
         SpanNearQueryBuilder queryBuilder = new SpanNearQueryBuilder(clauses.get(0), slop);
         for (int i = 1; i < clauses.size(); i++) {
             queryBuilder.addClause(clauses.get(i));

core/src/test/java/org/elasticsearch/index/query/SpanNearQueryBuilderTests.java

@@ -113,6 +113,28 @@ public class SpanNearQueryBuilderTests extends AbstractQueryTestCase<SpanNearQue
         assertEquals(json, false, parsed.inOrder());
     }
 
+    public void testParsingSlopDefault() throws IOException {
+        String json =
+                "{\n" +
+                "  \"span_near\" : {\n" +
+                "    \"clauses\" : [ {\n" +
+                "      \"span_term\" : {\n" +
+                "        \"field\" : {\n" +
+                "          \"value\" : \"value1\",\n" +
+                "          \"boost\" : 1.0\n" +
+                "        }\n" +
+                "      }\n" +
+                "    }]\n" +
+                "  }\n" +
+                "}";
+
+        SpanNearQueryBuilder parsed = (SpanNearQueryBuilder) parseQuery(json);
+        assertEquals(json, 1, parsed.clauses().size());
+        assertEquals(json, SpanNearQueryBuilder.DEFAULT_SLOP, parsed.slop());
+        assertEquals(json, SpanNearQueryBuilder.DEFAULT_BOOST, parsed.boost(), 0.0);
+        assertEquals(json, SpanNearQueryBuilder.DEFAULT_IN_ORDER, parsed.inOrder());
+    }
+
     public void testCollectPayloadsNoLongerSupported() throws Exception {
         String json =
                 "{\n" +